1. Field of the Invention
This invention relates generally to adjustment knobs and is concerned more particularly with a knob assembly having means for controllably locking a rotatable shaft in a desired angular position.
2. Discussion of the Prior Art
Control devices, such as valves, timers, rheostats, potentiometers, and the like, generally are provided with a protruding shaft which may be rotated to adjust the device to a desired setting. A coaxially disposed knob may be affixed to a distal end portion of the shaft such that rotation of the knob produces a corresponding rotation of the shaft. Also, for critically adjusted devices, for example, the knob may be provided with suitable locking means for maintaining the rotatable shaft in a selected angular position, until a new adjustment is required.
Thus, U.S. Pat. No. 2,787,353 granted to L. Sparagen on Apr. 21, 1957 discloses a push-to-turn knob assembly having a resiliently biased locking means for preventing inadvertent rotation of an adjustment shaft. The referenced knob assembly comprises a fixed race encircling the shaft and having therein pairs of resiliently spaced rollers which bindingly engage adjacent projections of an axially disposed cam cylinder. The cam cylinder surrounds the shaft and is provided with suitable means, such as a radial binding screw, for example, for fixedly engaging the shaft. Surrounding the cam cylinder is an axially coextensive, releasing cylinder having arcuate end portions disposed between respective adjacent pairs of the ball members within the race. The cam cylinder and the releasing cylinder extend axially beyond the distal end of the shaft and have adjacent end portions provided with respective diametrically aligned slots. The slots are aligned with a key extending axially from the closed end of a resiliently positioned knob shell which encloses the assembly.
In operation, the knob shell is pressed axially and rotated thereby causing the key to initially engage a side of the slot in the releasing cylinder and then a side of the slot in the cam cylinder. Accordingly, the slightest rotation of the releasing cylinder exerts a pressure on the resiliently spaced rollers in the race to move them out of binding engagement with the cam cylinder and unlock the attached shaft. Subsequent rotation of the releasing cylinder and the cam cylinder produces a corresponding rotation of the shaft whereby the control device is adjusted to a new setting. Releasing the knob shell withdraws the key from the slots thereby removing the pressure from the resiliently spaced rollers in the race. Consequently, the rollers exert a resilient counterpressure on the interposed arcuate portions of the releasing cylinder and move back into binding engagement with adjacent projections of the cam cylinder, thereby locking the attached shaft in the newly adjusted position.
However, it has been found that the referenced knob assembly may not operate as described due to a build-up of tolerances between the coaxially disposed inner cam cylinder, the intermediate releasing cylinder, and the cylindrical wall of the outer knob shell. Thus, the outer diameter of the cam cylinder may be oversized or the inner diameter of the releasing cylinder undersized sufficiently to produce frictional engagement therebetween. Similarly, the outer diameter of the releasing cylinder may be oversized or the inner diameter of the knob shell may be undersized sufficiently to produce frictional engagement. In either instance, it has been found that rotation of the knob shell, without applying an axial pressure thereto, may cause the releasing cylinder to exert enough pressure on the resiliently spaced rollers in the race to unlock the cam cylinder and produce a corresponding rotation of the shaft. Furthermore, it has been found that the described frictional engagement may prevent the resiliently spaced rollers from moving back into binding engagement with the cam cylinder and locking the shaft in a newly adjusted position. Consequently, the shaft may be rotated inadvertently, and the purpose of the push-to-turn knob assembly may be frustrated.
Therefore, it is advantageous and desirable to provide a push-to-turn knob assembly of the described type with a releasing member which avoids the problems of frictional engagement with the cam cylinder and the cylindrical wall of the knob shell.